**4. Valorization of sugarcane bagasse for bioethanol production**

and as an effective adsorbent material to remove toxic metals and dyes from wastewater [19, 20]. More recently, sugarcane bagasse has been used as a raw material to produce carbon quantum dots which can be used as biosensors in light-emitting diodes and even in drug delivery [21]. This chapter concerns the two most common applications of energy and bioethanol production

Burning or incineration in a boiler for steam generation is the most common application of bagasse using a cogeneration system for steam and power generation [22]. This allows supplying heat and power to the sugar and ethanol process and exporting any excess. In countries such as Brazil, where sugarcane industry is well developed, power generation has been largely supported by the government incentives and can be a major revenue component, after sugar and ethanol sales. **Figure 2** shows the two simplified typical cogeneration

The backpressure steam turbine (BPST) system in **Figure 2a** is more common. In this system, only the amount of bagasse necessary to match the heat required for the process is burned, thus leaving some excess bagasse that can be used for other purposes or needs to be disposed of. The steam is produced from water treated to remove some minerals and is called boiler feed water. The less efficient old systems generate steam at medium pressure of 22 bar and a temperature of 300°C, while the most modern systems can operate at up to 100 bar and 530°C [22]. The steam is then passed through the BPST with a discharge pressure of 2.5 bar and 140°C to meet the low-pressure steam required by the sugar refinery. The condensing

**Figure 2.** Typical cogeneration systems in sugarcane refineries using (a) a backpressure steam turbine (BPST) and (b) a

from sugarcane bagasse, which are described in the following sections.

**mills**

74 Sugarcane - Technology and Research

systems used.

condensing extraction steam turbine (CEST).

**3. Sugarcane bagasse incineration for energy generation in sugar** 

The use of sugarcane bagasse for bioethanol production has been extensively researched in recent years [23, 24]. The processing of sugarcane starts with the cleaning of sugarcane and extraction of sugars: juice treatment, concentration and sterilization [25]. Sugar extraction is carried out using mills to produce a sugarcane juice which follows a series of treatment, clarification and dewatering until the crystallization and centrifugation of sugar crystals. The production of ethanol from the juice, molasses or bagasse includes additional processing units of fermentation, that is, distillation and dehydration.

Ethanol can be prepared by the fermentation of molasses which contain 60% of fermentable sugars as described in [1]. Molasses is first diluted with water in 1:5 (molasses/water) ratio by volume. If molasses lack sufficient amount of nitrogen, it is fortified with ammonium sulfate to provide adequate supply of nitrogen to yeasts. Fortified solution of molasses is then acidified with a small quantity of sulfuric acid. The addition of acid favors the growth of yeasts and hinders the growth of unwanted bacteria. The resulting solution is then transferred to a large tank, and yeast is added to it at 30°C and left to ferment for 2–3 days. During this period, sucrase and zymase present in yeasts convert the sugars in molasses into ethanol according to the following simplified chemical reactions [26]:

$$\text{C}\_{12}\text{H}\_{22}\text{O}\_{11} + \text{H}\_{2}\text{O} \xrightarrow{\Delta} 2\text{C}\_{6}\text{H}\_{12}\text{O}\_{6} \tag{1}$$

$$\rm C\_6H\_{12}O\_6 \xrightarrow{} 2\rm C\_2H\_5OH + 2CO\_2 \tag{2}$$

The alcohol concentration in the fermentation broth is only 15–18%. The broth is sent to a distillation system to obtain 92% pure alcohol, also known as rectified spirit or commercial alcohol. A further purification step by molecular sieves or pervaporation is needed to produce anhydrous bioethanol for blending with gasoline.

An additional pretreatment step is needed in the production of bioethanol from bagasse. Pretreatment of the sugarcane bagasse is important because it helps to separate lignin and hemicellulose from cellulose, reduce cellulose crystallinity and increase the porosity of bagasse, thus improving cellulose hydrolysis [27]. Lignocelluloses are made up of three main polymer types: lignin encasing cellulose in cell walls provides rigidity of cell walls, hemicelluloses cover the cellulose and strengthen cell walls by interaction between lignin and cellulose, while encased cellulose microfibrils gives tensile strength to cell walls [28]. Celluloses and hemicelluloses are polysaccharides of C6 and C5 monomers, respectively, connected by β-(1–4)-glycosidic linkages. The main lignin compounds are polymers of para-hydroxyphenyl (H lignin), guaiacyl (G lignin) and syringyl (S lignin) alcohol. Pretreatment liberates hemicelluloses first because these are hydrolyzed at a faster rate. Liberation of hemicellulose separates lignin and cellulose. β-(1–4)-glycosidic linkages are broken down by pretreatment, liberating glucose from celluloses. The various methods for pretreatment of lignocellulosic materials such as sugarcane bagasse include acid hydrolysis, alkaline hydrolysis, steam or ammonia fiber expansion, organosolv, enzymatic hydrolysis, microwave and ultrasonication, and thereof combinations between these. The most common method is the dilute acid. Ozonolysis has also been used to pretreat sugarcane and agave bagasse [5].

**5. Energy balance and emissions**

second-generation bioethanol plant.

The current major use of sugarcane bagasse is for power supply in sugar refineries, making this facilities energy self-sufficient. Depending on the process configuration and energy requirements, some of them even export electricity to grid due to the excess bagasse available [23]. As commented in the previous section, an alternative use extensively researched nowadays is in bioethanol production [24]. In this section, the energy balance and emissions of the two alternative uses of bagasse are discussed. The indicator used to compare energy balance is the energy ratio which is defined as the energy output per unit of energy input. Energy input includes the energy originally contained in the bagasse based on its higher heating value. In the case of bagasse for power generation, the only input is the bagasse itself, in the case of the bioethanol production, the input also includes steam and electricity to run the

Sugarcane Bagasse Valorization Strategies for Bioethanol and Energy Production

http://dx.doi.org/10.5772/intechopen.72237

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To perform an energy balance using sugarcane for power generation, it is necessary to know the amount of steam and electricity required for the main sugar factory process. A typical electricity demand is 28 kWh/t cane and the process steam consumption of 500 kg/cane with low efficiency factory, or about 280–340 kg/t cane for modern efficient factories [33]. The balance also depends on the pressure at which the steam is generated and fed to the turbines. Using data from [22], the energy balance of a BST and CEST system on the basis of 1 ton of bagasse is shown in **Table 2**. Current efficiencies are quite low, only 20–24% and, as expected,

energy delivered. These values can be improved further through reduction of steam required in the sugar factory by better energy integration as well as by replacing old equipment with more efficient one. Highly efficient cogeneration systems can achieve up to more than 80% efficiency. Improvements can lead to a significant amount of surplus bagasse becoming available for other purposes such as production of bioethanol. In such a case, approximately 50%

Given the wide availability of bagasse as an agroindustrial residue, its use for bioethanol production has been widely investigated. The energy balance for this process may be less favorable as the ethanol yields can be relatively low and may require additional energy inputs. Strategies to achieve higher efficiencies in integrated systems combine (1) higher ethanol production can be achieved by the proper pretreatment and hexoses and pentoses fermentation

**System Ethanol yield (L/t bagasse) Reference** Pretreatment + enzymatic hydrolysis 149.3 [30] Two-stage dilute acid pretreatment + organosolv 192 [31] With pentoses also fermented to ethanol 335 [32]

emissions per GJ of

the CEST system performs better with higher energy ratio and lower CO2

of the bagasse is sufficient to supply the energy needs of sugar mills [33].

**Table 2.** Reported bioethanol yields from sugarcane bagasse.

**Table 1** shows values for bioethanol yields reported for various systems. The key to high ethanol yield is to enable the conversion of both hexoses and pentoses into ethanol. This requires the search for new microorganisms and their metabolic engineering. A leading second-generation bioethanol plant using sugarcane bagasse is operating in Brazil by the company Raizen, a joint venture between Shell and Cosan. This highly advanced integrated facility is able to boost bioethanol production by up to 50%, in addition to the first-generation plant and without expanding cultivation land use. The use of bagasse and straws allows production even during off-season for sugarcane harvest. The progressive scalingup has allowed producing 7 million liters in its first year and planned to reach a groundbreaking 40 million liters by 2018 [29].

Ethanol is used as an alternative energy source in top sugarcane-producing countries such as Brazil, India and China. World production of ethanol in 2013 was about 89 GL, with 74% of the world supply coming from Brazil and the USA [1]. The increasing biofuel production causes an increase in the biomass demand for energy purposes, which poses the challenge of the fuel versus food dilemma. The use of biomass has also raised some questions about the real benefits to decrease environmental impacts of the bioenergy systems that seek to replace fossil fuels due to the greenhouse gas emissions generated during crop cultivation and processing. To avoid unintended consequences and the translocation of issues of using biomass resources, a comprehensive analysis taking into account emissions and externalities related to energy and material consumption in the whole life cycle of sugarcane-based bioenergy systems is essential to ensure their sustainability.


**Table 1.** Energy ratio and direct process CO2 emissions for bagasse use in power generation.
